A number of methods have been developed for treating metal flake pigments to inhibit their reactivity with water, thereby rendering them usable in aqueous coatings and ink systems. Metal pigment particles readily react with water to generate hydrogen gas. This reaction is a safety hazard due to the generation of flammable hydrogen gas. Additionally, the reaction between the metal pigment particles and water degrades the aesthetic (optical) properties of the metal pigment particles by converting them to an oxidized form, reducing their bright, sparkling effect.
Due to the extremely high ratio of surface area to mass, metal pigment particles or flakes are much more reactive than bulk metal items. As such, less acidic organophosphorous reagents are generally used instead of phosphorous or phosphoric acid to treat the particles to inhibit their reactivity to water. These reagents typically include: phosphites (organic esters of phosphorous acid); phosphates (organic esters of phosphoric acid); phosphonic acids (where the organic moiety is directly bound to the phosphorus by a C—P bond); or neutralized versions of these materials. The treatments can be carried out by either adding the organophosphorous reagent to the coating or ink system prior to or immediately after adding the metallic flake pigment (referred to herein as an “in-situ treatment”), or by separately combining the organophosphorous reagent with the metal pigment and providing the treated flake paste to the coating or ink manufacturer (referred to as a “pre-passivation treatment”).
While phosphites, phosphates, phosphonic acids, and their respective neutralized versions have been shown to inhibit the reactivity of metallic flake pigments to water, their use has several drawbacks. For example, metal pigment particles treated with many of the reagents still result in the generation of fairly high levels of hydrogen gas when contacted with an aqueous medium, limiting their utility. This is particularly true for metal pigment particles treated with phosphates and phosphites. Additionally, when these reagents are used in large amounts to treat metal pigment particles in a coating, they can negatively affect other properties of the coating such as the coating's ability to adhere to a surface.
Many phosphonic acids and some phosphites and phosphates, when used as a pre-passivation treatment, due to the close proximity of the aluminum flakes to one another are often so reactive that the flakes are deteriorated. This leads to a reduction of brightness, loss of hiding power, and the formation of three-dimesional “seed” particles that protrude through the paint film. Neutralizing these aggressive materials with ammonia or with the low molecular weight amines commonly used in the coatings industry will, in some cases, prevent this deterioration, but in other cases it has no effect. Even when the neutralization is effective, it generally causes the inhibition reaction to proceed more slowly, so that an aging period of several weeks or even months is needed in order to develop the full benefits of the treatment. This aging period is undesirable in the manufacturing process of the metal pigment paste, as the metal pigment paste needs to be stored in a warehouse and periodically tested to ensure that it is ready for shipment to the end user.